While a number of recent efforts are being made to achieve a finer pattern rule in the drive for higher integration and operating speeds in LSI devices, the commonly used light exposure lithography is approaching the essential limit of resolution determined by the light source wavelength.
As the light source used in the lithography for resist pattern formation, g-line (436 nm) or i-line (365 nm) from a mercury lamp has been widely used. One means believed effective for further reducing the feature size is to reduce the wavelength of exposure light. For the mass production process of 64 M-bit DRAM, the exposure light, source of i-line (365 nm) was replaced by a KrF excimer laser having a shorter wavelength of 248 nm. However, for the fabrication of DRAM with a degree of integration of 1 G or more requiring a finer patterning technology (processing feature size 0.13 μm or less), a shorter wavelength light source is required. In particular, photolithography using ArF excimer laser light (193 nm) is now under investigation.
On the other hand, it is known in the art that the bilayer resist process is advantageous in forming a high-aspect ratio pattern on a stepped substrate. In order that a bilayer resist film be developable with a common alkaline developer, high molecular weight silicone compounds having hydrophilic groups such as hydroxyl and carboxyl groups must be used.
Among silicone base chemically amplified positive resist compositions, recently proposed were those compositions for KrF excimer laser exposure comprising a base resin in the form of polyhydroxybenzylsilsesquioxane, which is a stable alkali-soluble silicone polymer, in which some phenolic hydroxyl groups are protected with t-BOC groups, in combination with an acid generator (see JP-A H06-118651 and SPIE vol. 1925 (1993), 077). For ArF excimer laser exposure, positive resist compositions comprising as a base a silsesquioxane of the type in which cyclohexylcarboxylic acid has substituted thereon an acid labile group were proposed (see JP-A H10-324748, JP-A H11-302382, and SPIE vol. 3333 (1998), p 62). For F, laser exposure, positive resist compositions based on a silsesquioxane having hexafluoroisopropanol as a dissolvable group were proposed (see JP-A 2002-55456). The above polymer bears in its backbone a polysilsesquioxane containing a ladder skeleton produced through polycondensation of a trialkoxysilane or trihalosilane.
Silicon-containing (meth)acrylate polymers were proposed as a resist base polymer having silicon pendants on side chains (see JP-A H09-110938, J. Photopolymer Sci. and Technol., Vol. 9, No. 3 (1996), p 435-446).
The lower (or bottom) layer of the bilayer resist process is formed of a hydrocarbon compound which can be etched with oxygen gas, and must have high etch resistance since it serves as a mask when the underlying substrate is subsequently etched. For oxygen gas etching, the bottom layer must be formed solely of a silicon atom-free hydrocarbon. To improve the line-width controllability of the upper (or top) layer of silicon-containing resist and to minimize the sidewall corrugation and pattern collapse by standing waves, the bottom layer must also have the function of an antireflective coating (ARC). Specifically, the reflectance from the resist bottom layer back into the resist top layer must be reduced to or below 1%.
Now, the results of calculation of reflectance at film thickness varying up to the maximum of 500 nm are shown in FIGS. 2 and 3. Assume that the exposure wavelength is 193 nm, and the resist top layer has an n value of 1.74 and a k value of 0.02. FIG. 2 shows substrate reflectance when the resist bottom layer has a fixed k value of 0.3, the n value varies from 1.0 to 2.0 on the ordinate and the film thickness varies from 0 to 500 nm on the abscissa. Assuming that the resist bottom layer of the bilayer resist process has a thickness of 300 nm or greater, optimum values at which the reflectance is reduced to or below 1% exist in the refractive index (n) range of 1.6 to 1.9 which is approximate to or slightly higher than that of the resist top layer.
FIG. 3 shows substrate reflectance when the resist bottom layer has a fixed n value of 1.5 and the k value varies from 0 to 0.8. Assuming that the resist bottom layer of the bilayer resist process has a thickness of at least 300 nm, the reflectance can be reduced to or below 1% as long as the k value is in a range of 0.24 to 0.15. By contrast, the antireflective coating used in the form of a thin film of about 40 nm thick in the single-layer resist process has an optimum k value in the range of 0.4 to 0.5, which differs from the optimum k value of the resist bottom layer used with a thickness of 300 nm or greater in the bilayer resist process. For the resist bottom layer in the bilayer resist process, a film having a lower k value, that is, more transparent is necessary.
As the material for forming a resist bottom layer in 193 nm lithography, copolymers of polyhydroxystyrene with acrylates are under study as described in SPIE Vol. 4345 (2001) p 50. Polyhydroxystyrene has a very strong absorption at 193 nm and its k value is as high as around 0.6 by itself. By copolymerizing it with an acrylic compound having a k value of almost 0, the k value of the copolymer is adjusted to around 0.25.
However, the resistance of the acrylic polymer to substrate etching is weak as compared with polyhydroxystyrene, and a considerable proportion of the acrylic compound must be copolymerized in order to reduce the k value. As a result, the resistance to substrate etching is considerably reduced. The etch resistance is not only reflected by the etching speed, but also evidenced by the development of surface roughness after etching. Through copolymerization of acrylic compound, the surface roughness after etching is increased to a level of serious concern.
Also proposed was a tri-layer process of stacking a resist top layer of a silicon-free single-layer resist film, a resist middle layer containing silicon below the top layer, and a resist bottom layer of organic film below the middle layer. See J. Vac. Sci. Technol., 16(6), November/December 1979. Since the single-layer resist generally provides better resolution than the silicon-bearing resist, the tri-layer process permits such a high resolution single-layer resist to be used as an imaging layer for light exposure. A spin-on-glass (SOG) coating is used as the resist middle layer. A number of SOG films have been proposed.
In the trilayer process, the optimum optical constants of the bottom layer for controlling reflection from the substrate are different from those in the bilayer process. The purpose of minimizing substrate reflection, specifically to a level of 1% or less is the same between the bi- and tri-layer processes. In the bilayer process, only the resist bottom layer is endowed with the antireflective effect. In the tri-layer process, either one or both of the resist middle layer and resist bottom layer may be endowed with the antireflective effect.
U.S. Pat. No. 6,506,497 and U.S. Pat. No. 6,420,088 disclose silicon-containing layer materials endowed with antireflective effect. In general, a multi-layer antireflective coating has greater antireflective effect than a single-layer antireflective coating and is commercially widely used as an antireflective film for optical articles. A higher antireflective effect is obtainable by imparting an antireflective effect to both a resist middle layer and a resist bottom layer. If the silicon-containing resist middle layer in the trilayer process is endowed with the function of ARC, the resist bottom layer need not necessarily possess the maximum function of ARC as in the case of the bilayer process. In the trilayer process, the resist bottom layer is required to have high etch resistance during substrate processing rather than the ARC function. Then a novolac resin containing more aromatic groups and having high etch resistance has been used as the resist bottom layer in the trilayer process.
FIG. 4 illustrates substrate reflectance with a change of the k value of the resist middle layer. It is seen that by setting a k value as low as 0.2 or less and an appropriate thickness to the resist middle layer, a satisfactory antireflective effect as demonstrated by a substrate reflectance of up to 1% is achievable. In general, the ARC film must have a k value of 0.2 or greater in order to reduce reflectance to or below 1% at a film thickness of 100 nm or less (see FIG. 3). In the trilayer resist structure wherein the resist bottom layer serves to restrain reflection to a certain extent, the resist middle layer may have an optimum k value of less than 0.2.
FIGS. 5 and 6 illustrate changes of reflectance with the varying thickness of the resist middle layer and resist bottom layer, when the resist bottom layer has a k value of 0.2 and 0.6, respectively. The resist bottom layer in FIG. 5 has a k value of 0.2 which assumedly corresponds to the resist bottom layer optimized for the bilayer process, and the resist bottom layer in FIG. 6 has a k value of 0.6 which is approximate to the k values at 193 nm of novolac and polyhydroxystyrene. The thickness of the resist bottom layer varies with the topography of the substrate whereas the thickness of the resist middle layer is kept substantially unchanged so that presumably it can be coated to the predetermined thickness.
The resist bottom layer with a higher k value (0.6) is effective in reducing reflectance to 1% or less with a thinner film. In the event that the resist bottom layer has a k value of 0.2 and a thickness of 250 nm, the resist middle layer must be increased in thickness in order to provide a reflectance of 1% or less. Increasing the thickness of the resist middle layer is not preferable because a greater load is applied to the resist film as the uppermost layer during dry etching of the resist middle layer.
FIGS. 5 and 6 illustrate reflection during dry exposure through an exposure tool having a lens with a NA of 0.85, indicating that by optimizing the n and k values and thickness of the resist middle layer for the trilayer process, a reflectance of up to 1% is achievable independent of the k value of the resist bottom layer. Nevertheless, with the advance of the immersion lithography, the NA of the projection lens increases beyond 1.0, and the angles of light entering not only the resist film, but also the underlying ARC film become smaller. The ARC film serves to control reflection due to the absorption of the film itself and the offsetting effect by optical interference. Since oblique light produces a less optical interference effect, reflection increases. Of the films in the trilayer process, it is the resist middle layer that provides reflection control by utilizing the optical interference effect. The resist bottom layer is too thick to utilize the optical interference effect and lacks the anti-reflective function due to the offsetting effect by optical interference. It is necessary to control the reflection from the surface of the resist bottom layer. To this end, the resist bottom layer must have a k value of less than 0.6 and an n value approximate to that of the overlying, resist middle layer. If a film has a too small value of k and too high transparency, reflection from the substrate also occurs, and a k value of about 0.25 to 0.48 is optimum in the case of immersion lithography at NA 1.3. With respect to the n value, a value approximate to the resist's n value of 1.7 is the target for both the middle and bottom layers.
Since benzene ring structure has very strong absorption, cresol novolac resins and polyhydroxystyrene resins containing the same have k values in excess of 0.6. Naphthalene ring structure is one of structures having higher transparency at wavelength 193 nm and higher etch resistance than the benzene ring. For example, JP-A 2002-014474 discloses a resist bottom layer comprising a naphthalene or anthracene ring. According to the inventors' measurements, naphthol co-condensed novolac resin and polyvinylnaphthalene resin have a k value between 0.3 and 0.4. Also the naphthol co-condensed novolac resin and polyvinylnaphthalene resin have a low n value at wavelength 193 nm, specifically, the n value is 1.4 for the naphthol co-condensed novolac resin and as low as 1.2 for the polyvinylnaphthalene resin. Acenaphthylene polymers disclosed in JP-A 2001-040293 and JP-A 2002-214777, for example, have a n value of 1.5 and a k value of 0.4 at 193 nm, close to the target values. There is a need for a bottom layer having a high n value, a low k value, transparency and high etch resistance. Notably JP-A 2007-199653 discloses a resist bottom layer material having a bisnaphthol group, the material having n and k values close to the target values, and improved etch resistance.
If the underlying processable substrate has steps, it is necessary to deposit a resist bottom layer to planarize the steps. By the planarization of the resist bottom layer, a variation in thickness of an overlying film, which may be a resist middle layer or a resist top layer or photoresist film, is minimized, and the focus margin of lithography can be enlarged.
When an amorphous carbon bottom layer is formed by CVD using a reactant gas such as methane, ethane or acetylene gas, it is difficult to bury steps to be flat. On the other hand, when a resist bottom layer is formed by spin coating, there is a benefit that irregularities on the substrate can be buried. Suitable means for improving the burying properties of a material of coating type include the use of a novolac resin having a low molecular weight and a broad molecular weight distribution as disclosed in JP-A 2002-047430 and a blend of a base polymer and a low molecular weight compound having a low melting point as disclosed in JP-A H11-154638.
It is known from SPIE vol. 469, p 72 (1984) that novolac resins cure through intermolecular crosslinking merely by heating. Reported therein is a crosslinking mechanism by radical coupling that upon heating, a phenoxy radical generates from a phenolic hydroxyl group of cresol novolac resin, and the radical migrates to methylene, a linking group of the novolac resin via resonance, whereby methylene moieties crosslink together. JP 3504247 discloses a pattern forming process using a bottom layer having a carbon density which is increased by thermally induced dehydrogenation or dehydration condensation reaction of polycyclic aromatic compounds such as polyarylene, naphthol novolac, and hydroxyanthracene novolac.
A vitreous carbon film is formed by heating at or above 800° C. (see Glass Carbon Bull. Chem. Soc. JPN, 41 (12) 3023-3024 (1968)). However, the upper limit of the temperature to which the wafer can be heated by the lithography wafer process is up to 600° C., preferably up to 500° C. when thermal impacts like device damage and wafer deformation are taken into account.
It is reported in Proc. of Symp. Dry. Process, p 11 (2005) that as the processing line width is reduced, the resist bottom layer can be twisted or bowed when the processable substrate is etched using the resist bottom layer as mask. During etching of the substrate with fluorocarbon-based gas, a phenomenon occurs that hydrogen atoms in the resist bottom layer are replaced by fluorine atoms. As the surface of the resist bottom layer is converted to fluorocarbon-like, the bottom layer increases its volume so that it may swell or lower its glass transition temperature, allowing a finer pattern to be twisted. It is described in the literature that twisting can be prevented by applying a resist bottom layer having a low hydrogen content. An amorphous carbon film formed by CVD is effective for preventing twist because the hydrogen content of the film can be minimized. However, the CVD has poor step burying properties as pointed out above, and the CVD apparatus may be difficult to introduce because of its price and footprint area. If the twist problem can be solved by a bottom layer material from which a film can be formed by coating, specifically spin coating, significant merits would result from simplification of process and apparatus.
Also under study is a multilayer process in which a hard mask is formed on the resist bottom layer by the CVD technique. In the case of silicon-based hard masks (such as silicon oxide, silicon nitride, and silicon oxynitride films) as well, inorganic hard masks formed by CVD or similar deposition techniques have more etch resistance than hard masks formed by the spin coating technique. In the event the processable substrate is a low-dielectric-constant film, the photoresist may be poisoned therefrom (poisoning problem). The CVD film is more effective as a barrier film for preventing the poisoning problem.
Then a process involving forming a resist bottom layer by spin coating for planarization purpose, and forming an inorganic hard mask middle layer as the resist middle layer by a CVD technique is investigated. When an inorganic hard mask middle layer, especially a nitride film, is formed by a CVD technique, the substrate must be heated at a temperature of at least 300° C., typically about 400° C. Accordingly, when the resist bottom layer is formed by spin coating, the substrate must have heat resistance at 400° C. Ordinary cresol novolac resins, naphthol novolac resins, and even fluorene bisphenol resins known to be heat resistant fail to withstand heat at 400° C., experiencing a substantial film slimming after heating. There is a need for a resist bottom layer which can withstand heating at high temperature when an inorganic hard mask middle layer is formed by a CVD technique.
Because of the problem of film slimming or resin degradation after heating due to shortage of heat resistance, heat treatment of a resist bottom layer material is usually carried out at or below 300° C., typically 80 to 300° C. The heat treated film, however, still suffers from slimming after solvent treatment or twisting of the pattern during etching of the substrate.
As discussed above, it would be desirable to have a method for forming a resist bottom layer which has optimum values of n and k as the ARC film, burying properties, etching resistance, and solvent resistance, and has sufficient heat resistance to withstand high temperature encountered during formation of an inorganic hard mask middle layer by a CVD or similar deposition technique, and prevents pattern twisting during substrate etching.